This invention relates to pressurized cans from which a fluent product is dispensed by actuating a product discharge valve, and particularly, a pressurized can having a rolling diaphragm or barrier which separates the product from a pressurized gaseous or liquefied propellant.
Pressurized cans are used for dispensing liquid, semiviscous and viscous products. A can from which a liquid product is dispensed is often called an aerosol can. In some of these cans, in order to prevent cavitation, a barrier separates the product from the propellant. Three basic types of barriers in pressurized cans have typically been used, a piston system, a sprayed on strippable film bag, or a bag system.
In the piston system, a free piston, which is shiftable along the interior of the can, is the barrier. See U.S. Pat. No. 4,171,757. The piston system works for many products, but because the piston does not create an impenetrable barrier at the can wall, this system should not be used for products which may bypass the piston. Furthermore, the piston system is also ineffective with certain limited types of seamed cans, oddly shaped cans, cans that change in cross-section over the height of the can, and misshapen cans, since the barrier piston then has difficulty sealing to the wall of the can as the piston moves. The piston also may not seal correctly in soft or flexible walled or deformed cans.
In the strippable film system, a plastic composition is sprayed onto the peripheral side wall and the bottom wall of the can. As the product is expelled from the can, the film is pushed up by the pressurized propellant beneath it, and the film gradually strips away from the sides and bottom of the can to push the product out. Because the bag is being stripped away from the bottom upwardly, the bag cannot be "pinched-off" and a cut off in the flow of the product is avoided. This arrangement has a more expensive fabricating process.
The bag system has a number of variants. In one variant, a bag is inserted into the can and it is either brought out and around the lip of the can or it is sealed to the chime or top rim of the can. In either case, special folds or pleats formed in the bag or a collecting tube in the bag are necessary to prevent the bag from collapsing and pinching or cutting off the flow of the product, especially as the bag collapses toward the top of the can under pressure while the product is being expelled. The bag system tends to be expensive because the bags have to be made with either folds or pleats to avoid the "pinching-off" problem. A further disadvantage of a bag system is that bags which are connected at their opening to the lip or chime of the cans tend to both collapse and tear off at the chime or at the seams. Although inserting a collecting tube into the bag may overcome some of these problems, the increased cost tends to make this approach impractical. In a modification of the just described bag system, the bag is simply secured at the top or the bottom of the can, without being a specially designed bag, and this does not have a capacity of fully expelling all of the contents of the can.
In another variant of the bag system, the bag is fixedly secured part way along the height of the can, between the ends of the can. In typical examples of this system, the position of the bag along the height of the can is predetermined, before can assembly and filling, by the bag being secured between bottom and top halves of a two part container, by an attachment fixture in the can, or by slots or grooves in the can which fix the location of the bag. Such a bag may be capable of everting for expelling all of the contents of the can. But, this variant is not universally efficient for all pressures or all materials being expelled, for all types of propellants or all sizes of cans, and assembly of a can with such a bag system may be difficult or expensive.
Different propellants, e.g. a gaseous propellant or a liquid propellant, require that they occupy quite different percentages of the total volume of a can, as discussed in more detail below. For any particular size can, where the position of the bag along the can is predetermined by the can design, it is necessary for a manufacturer to design and inventory different sets of cans for differently positioned bags in the cans. A can which is more universally usable would be preferred.
Where a can is pressurized, this would tend to expand its side walls, that is, unless they were relatively thick and rigid and thus more expensive than thinner walled, lighter weight cans. Also, the barrier to can wall seal would end upon can wall expansion.